DeepBlue21

Sorry if this question has been asked before but has anyone done the math on when it makes sense to stop putting Yemen into levelling rewards girls during a run? Intuitively it makes sense to me to buy heavily during the early part of the run (compound more increased rewards over more levels) but at some point the incremental investment won't pay back before the end of the run right? I've guesstimated that upgrades don't make sense once you're 80-90% through the run but just curious if anyone has done a more thoughtful calculation?